fix: convert buffers into blobs before calling blobStore.set

[mrstork]

Summary

Fixes typescript issue in main branch that's blocking test runs

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[serhalp]

This might be worth a quick manual test before releasing it to buildbot, just to be extra safe. If you link this into netlify/cli locally and deploy a Next.js site, that should exercise this code path. Ping me if you need help with that 🙂.

[mrstork]

Replaced by #5927